The Importance of Polyploidy in Evolution

Polyploidy, particularly allopolyploidy, often gives rise to new species and has been particularly important in the evolution of flowering plants. Occasional genome doubling through polyploidy has been a major contributor to evolutionary success in several groups. For example, Saccharomyces cerevisiae (yeast) is a tetraploid, having undergone whole-genome duplication about 100 million years ago. The vertebrate genome has duplicated twice, once in the common ancestor of jawed vertebrates and again in the ancestor of fishes. Certain groups of vertebrates, such as some frogs and some fishes, have undergone additional duplications. Cereal plants have undergone several genome duplication events. The types of chromosome mutations we have discussed in this chapter are summarized in Table 6.3.

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TABLE 6.3 Different types of chromosome mutations
Chromosome mutation Definition

Chromosome rearrangement

 Chromosome duplication

 Chromosome deletion

 Inversion

  Paracentric inversion

 

  Pericentric inversion

 Translocation

 

  Nonreciprocal translocation

 

 

  Reciprocal translocation

Change in chromosome structure

Duplication of a chromosome segment

Deletion of a chromosome segment

Deletion of a chromosome segment

Inversion that does not include the centromere in the inverted region

Inversion that includes the centromere in the inverted region

Movement of a chromosome segment to a nonhomologous chromosome or to another region of the same chromosome

Movement of a chromosome segment to a nonhomologous chromosome or to another region of the same chromosome without reciprocal exchange

Exchange between segments of nonhomologous chromosomes or between regions of the same chromosome

Aneuploidy

 Nullisomy

 Monosomy

 Trisomy

 

 Tetrasomy

Change in number of individual chromosomes

Loss of both members of a homologous pair

Loss of one member of a homologous pair

Gain of one chromosome, resulting in three homologous chromosomes

Gain of two homologous chromosomes, resulting in four homologous chromosomes

Polyploidy

 Autopolyploidy

 

 Allopolyploidy

Addition of entire chromosome sets

Polyploidy in which extra chromosome sets are derived from the same species

Polyploidy in which extra chromosome sets are derived from two or more species

CONCEPTS

Polyploidy is the presence of extra chromosome sets. Autopolyploids possess extra chromosome sets from the same species; allopolyploids possess extra chromosome sets from two or more species. Problems in chromosome pairing and segregation often lead to sterility in autopolyploids, but many allopolyploids are fertile.

image CONCEPT CHECK 7

Species A has 2n = 16 chromosomes and species B has 2n = 14. How many chromosomes would be found in an allotriploid of these two species?

  1. 21 or 24

  2. 42 or 48

  3. 22 or 23

  4. 45

c